When I was a kid, my dad used to pour or spray used oil on our gravel driveway to keep the dust down in the summertime. It was great and killed the weeds, too . ... . yeah, UH Oh! Sadly, until about the last 20 or 30 years people really just didn't think about soil contamination, and the resulting water contamination that comes along with it. My dad was a doctor with 4 kids of his own, and our cabin (where the said driveway is) was on the shore of a beautiful pristine lake.
So a shed that is 112 years old could have had oil spilled or poured on the dirt floor, along with any number of other contaminants that we wouldn't dream of either using or pouring on the ground nowadays. Especially on clay soil that doesn't drain well, and has been covered by the shed for all these years, all that stuff could still be hanging around. As someone already said, I'd hesitate to grow any vegetables in that area. Keep it for your ornamentals.
If you want quick, efficient raised beds have you looked at using Earth Boxes? I have enormous oak trees on all 4 corners of our property and have discovered in the past 5 years that the oak trees have roots underneath just about every inch of soil in the yard. So my wonderful 3ft.X 12ft. raised bed in the middle of the sunniest part of the yard is now completely overrun with oak tree roots and won't grow much of anything until I can hire someone with a strong back to dig it out completely, sift the soil and re-fill it with compost.
So, well I have been lured to the Dark Side and now grow all my veggies in Earth Boxes supported above the ground by cinder blocks. They're extremely water efficient and productive, and since I put in an automatic micro-irrigation system for them I can even go out of town and not worry. Once I've planted them and started the automatic system all I have to do is make sure there is enough water - increase as the weather warms - and wait to harvest. Not as much bending over to prepare them, no weeding, and harvesting at eye level are all huge bonuses.
